The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Navy.
This invention relates to electric discharge lasers and more particularly to an improved discharge configuration adapted for use with electric discharge lasers having a transverse flowing gas medium.
Electric discharge lasers having a flowing gas gain medium are well known in the art. The basic motivation for employing a flowing gas gain medium for convective cooling of the laser discharge gases, especially for molecular discharge systems, and the benefits derived therefrom, such as enhanced laser performance, are taught by Brown et al in U.S. Pat. No. 3,641,457 filed Sept. 10, 1969 and held with the present application by a common assignee. Brown et al discloses the importance of maintaining the proper ratio of the electric field to the neutral gas density in the laser discharge to maximize energy transfer from the discharge electrons to the upper energy level of the lasing gas. Furthermore, the optical power output of a molecular discharge laser is porportional to the difference between the population densities of the upper energy level and the lower energy level of the lasing gas. As is disclosed by Brown et al, the population density of the upper energy level is directly enhanced by operating at pressures exceeding several torr, with greater enhancements being achieved as the pressure approaches atmospheric. By employing a flowing gas gain medium to convectively cool the discharge gases, the population density of the lower energy level is reduced which favorably influences laser performance.
While these basis principles influencing electric discharge laser performance have been demonstrated in a geometry in which the directions of the flowing gas, the electric field, and the optical axis are mutually coaxial, the resulting total system size and weight is not optimum. This occurs primarily because of the high pressure drop along the flow path, typically on the order of several meters, of the gas through the laser cavity. Pressure drop considerations in overall laser system design, especially when a closed cycle recirculating gas loop is employed, significantly impact the recirculating pump size, and therefore, overall laser system size, weight and cost. These considerations in many applications have lead to the development of alternate laser discharge and cavity geometries.
One configuration which is well known in the art is the electron beam stabilized transverse laser geometry. In this configuration, the flow direction, the discharge electric field direction, and the optical axis are mutually perpendicular. Typically, the discharge dimension both in the flow direction and transverse to both the flow direction and optical axis is short in comparison to the total optical path length. This results in a requirement for having discharge electrodes having large extended areas. To provide a uniform discharge over the extended area of the electrode at high pressures and to prevent the flow field from convecting the discharge out of the laser cavity in the downstream flow direction, a high energy electron beam is employed to produce a low level of ionization of the gas uniformly throughout the laser discharge region. At high pressures (greater than fifty torr), the low level of ionizaton provided by the electron beam moderates the onset of discharge instabilities which cause the desired diffuse discharge to collapse catastrophically into an arc.
As is well known in the art, arc formation destroys the laser output because the entire uniformly distributed diffuse electron current flow of the discharge is concentrated into the very small region of the arc in which the electric field to neutral gas density ratio is no longer maintained at the proper level for optimum population of the upper energy level of the laser gas. In addition, significant gas heating in the arc channel occurs which increases the lower energy level population, further destroying laser performance.
A transverse discharge configuration exhibits a very small flow impedance in the flow field due to discharge electrodes and/or optical elements and the discharge dimension in the flow direction is short, on the order of ten to twenty centimeters. Accordingly, the pressure drop across the laser cavity at high flow velocities is minimized. Also, in a transverse discharge configuration, the combined high flow velocity and short discharge length in the flow direction reduces gas residence time within the discharge. As taught by Nighan et al in U.S. Pat. No. 4,016,448 filed Nov. 26, 1975 and held with the present application by a common assignee, there is an inverse relationship between the maximum power density that can be deposited into the laser discharge before the onset of arc formation and the time the gas remains in the discharge region. Therefore, the transverse laser discharge configuration in addition to offering the potential for low pressure drop operation also offers the potential capability of extending to higher levels the maximum power density that can be deposited into the discharge prior to the onset of arcing.
Indeed, quite impressive laser performance has been achieved with transverse gas discharge laser systems employing high energy electron beam stabilization of the discharge. However, the use of a high energy electron beam represents a significant addition to the total number of laser system components, and increases the operational complexity and total laser system cost. For many laser applications these additions are prohibitive. As a consequence, alternate methods for the stabilization of the discharge are desired for systems having the discharge transverse to the path of the gas flow. A transverse discharge configuration in which both the discharge electric field and optical axis are transverse to the flow path of the gas is disclosed by Bullis et al in U.S. Pat. No. 3,743,963 filed Sept. 10, 1969 and held with the present application by a common assignee, in which the discharge has been stabilized through the use of Radio Frequency (RF) auxiliary ionization techniques. This approach, which is similar in nature to electron beam stabilization, utilizes a RF source to produce a weakly ionized uniform plasma in the discharge region. The weakly ionized plasma results in a uniform diffuse glow discharge across discharge electrodes having an extended area and stabilizes the discharge against flow field effects which normally would sweep the discharge downstream in the flow direction. While RF discharge stabilization represents a reduction in laser system complexity and cost, compared to electron beam stabilization, additional system components are still required with this approach.
One method to eliminate the additional complexities represented by the use of either electron beam or RF discharge stabilization is disclosed by Foster et al in U.S. Pat. No. 3,772,610 filed on Sept. 20, 1971 which teaches the use of an extended length transverse discharge configuration in which the discharge is allowed to be swept downstream by the convective forces of the flow field. In this configuration, an elongated cylindrical tubular cathode is disposed within the gas stream transverse to the gas path and upstream of a segmented anode disposed flush within a channel wall. The single tubular cathode produces turbulence in the region immediately adjacent to the downstream side of the cathode surface to provide a homogeneous gaseous medium for the purpose of enhancing discharge stabilization. A single pin electrode disposed proximate to the upstream side of the tubular cathode provides start-up ionization in the gas between the pin and the cathode. Ballast resistance is employed on each of the flush mounted segmented anode elements for discharge stabilization. While this configuration attempts to take advantage of the benefits to be derived from transverse discharge operation, failure to stabilize the discharge against convective flow of the gases results in only a limited range of conditions over which the discharge can be successfully operated. Specifically, because of the forces within the convective flow, discharge operation, as taught by Foster et al, is limited to pressures of approximately 35-50 torr. A second limit, which also determines the maximum operation pressure and velocity levels, is the requirement in this configuration to produce a uniform discharge over the elongated tubular cathode without the use of auxiliary ionization for discharge stabilization. Further, because the maximum operating gas flow velocities are considerably lower than those achievable with other transverse discharge configurations, the gas residence time is significantly longer which limits the maximum attainable discharge power density before the onset of discharge arcing.
At the May, 1977 IEEE/OSA Conference on Laser Engineering and Application, Wutzke et al disclosed a pin-to-plane transverse gas discharge laser geometry in which significant improvements in discharge performance were achieved over the Foster et al configuration. In the pin-to-plane transverse discharge geometry, the cathode structure, consisting of an array of small diameter pins located on one centimeter centers in several rows along the flow direction is disposed opposite to a planar anode and transverse to the gas path. To stabilize the discharge, each cathode pin is individually ballasted with a moderate to large value of resistance. This discharge configuration is capable of producing a stable discharge at pressures of 140 torr and discharge power densities up to 50 w/cm.sup.3. This performance represents approximately a factor of four increase in discharge operating pressure and approximately a factor of ten increase in discharge power density achievable over that reported by Foster et al. Furthermore, these increases were achieved at flow velocities of 150 m/sec which permits enhanced convective cooling of the discharge while also minimizing residence time of the discharge gas.
The discharge geometry disclosed by Wutzke et al has several system drawbacks. The requirement to provide individual ballasting of tens of thousands of ohms to each of the multiplicity of cathode pins results in a complex and costly cathode structure. Further, because of the magnitude of the ballast required, a significant amount of power is dissipated in the resistive ballast. This obviously impacts unfavorably on overall system efficiency. In addition, the discharge produced by the pin cathode configuration yields a highly concentrated plasma region adjacent to the pin. This results in relatively nonuniform discharge excitation along the optical axis wherein the regions immediately adjacent to the pin cathodes have discharge power densities approximately of an order of magnitude higher than the average for the whole discharge. As a consequence, the limiting power density prior to the onset of discharge arcing is determined by the power density in these localized regions rather than the average power level deposited in the total discharge. Because of this factor the average discharge power density is limited to levels significantly below those that would be determined by gas discharge residence time consideration.
Chenausky et al in U.S. patent application having Ser. No. 864,961, filed Dec. 27, 1977, and of common assignee herewith discloses an electrode configuration for the deposition of electrical power in a direction transverse to a path of flowing gas having a cathode comprising at least one row of pins in combination with at least one rod disposed downstream of and parallel to the row of pins and a planar anode disposed in a spaced apart relationship to the pins and rods. The pins are utilized as preionization electrodes and each pin is connected to a ballast resistance. The rods disposed downstream of the pins are utilized for electrical power deposition into the discharge and are unballasted which eliminates the power consumption within the electrical circuit of the power electrodes of prior art devices. At least sixty percent of the electrical power deposited into the discharge passes through the unballast power electrodes with the remaining portion of the power passing through the ballasted preionization electrodes. This electrode configuration enables high electrical power density deposition into the gas stream without the addition of complex and costly discharge stabilization elements.